Well there is a problem here. There is a setting in the registry (you can search for that string to find it) but it is also kept someplace else too. And to make matters worse, Windows may end up changing that value back on a reboot or update. An OS can totally be changed, but it requires some tricks and hex editing. You can find this info in the Customizing Windows section. As an example, I totally reskinned Server 2003 to look like XP Embedded, including what you are looking to do, and more like replacing the default OS screensaver. Here is an example: